Idling after coming back from a ride, I put it in neutral for a minute, the engine started to rev up to 3000 then down to 2000 then up to 3500 then down to 1500 as if I was doing it, but my foot was completely off the gas pedal. It happened 3-4 times between yesterday and today. Any idea? Thanks in advance

OK, I have a 94 Galant with about 140,000 miles on it. This past month something really weird has happened to me three times that I'm hoping so Arsians out there can clue me in on.
My car normal idles at ~600rpm, put it in neutral this is where it sits or if you're stopped at a light. Well three times now when I started it up the engine starts revving. It will rev up and down between 1,100 and 1,600rpm. It will shoot up to 1,600, drop down and as soon as it hits 1,100 it shoots back up to 1,600, over and over. It does this when it's in park or neutral. If I sit in drive at a light it refuses to drop below 1,100rpm.
It just randomly happens, I'll start up my car to go to eat dinner and it does it, when I'm done with dinner and go home it's fine. I've tried turning it off and then back on immediately when it happens and it still does it. I've tried revving the engine in park and that doesn't affect anything. The only thing that seems to work is time, if I let it sit for 30+ minutes, next time I start it there is a very good chance it won't happen. However, the times it has happened my car has been sitting anywhere from 30 minutes to 4 hours. Like I said it's random.
Anyone have this happen to them? Anyone know why it's happening? Any advice is appreciated.

You most likely have a transmission that has seen it's better days, very common issue in these models.

Check for vacuum leaks. With the rpm difference that large it should be pretty easy to hear the vacuum leak hissing with the engine running.

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Your car is probably in the limp mode meaning it won't allow you to rev the engine with it in gear. I suspect something majorly is wrong with your tranny. Limp modes are put into effect when you car senses trouble so it may still be fixable without too any $$$. When was the last time you serviced the transmission ( oil and filter change?)

Oxigen sensor. Check accelerator pot also, and fuel pump.

idle air control valve is bad replace it. at start up choke control runs till warm up while driving throttle opening controls engine at idle ,the idle air control valve.

if you have'nt replaced those spark plugs and wires lately, you need to.and also, lucas fuel treatment, in my personal opinion,is the very best for unclogging fuel lined and injectors.it being below a 1999 model and your check engine light is on, you can have it coderead for free if you're near anautozone or advance automotive parts store. they will code read it and tell you why that check engine lights coming on.hope this helps and thank you for choosing fixya.com

First- lets change engine coolant sensor/ECT/ and you will be very surprised.Black head with four pins.Be very carefull when change it because there is pressure into coolant system.First-open slowly the cap of engine coolant tank.Then tighten again to safe coolant.Unplug the sensor,remove the pin and pull out the sensor.If the "O"ring/old one/ is still inside-take it out.

Sounds like your "speed sensor" is failing. When it fails it will cause irregular shift patterns and engine idle speeds.